1. Field of the Invention
The present invention relates to a video signal separator, and more particularly, to a method and related apparatus for separating a luminance signal from a video signal.
2. Description of the Prior Art
It has been over fifty years since color TVs were introduced to the public. A composite color TV signal (video signal in brief) for a color TV includes luminance as well as chrominance information. A luminance signal of the video signal works with frequencies ranging from 0 (DC) to 5.5 MHz, while a chrominance signal, which is carried by a carrier, works from 3.58 MHz to 4.43 MHz and overlaps the luminance signal. Numerous patents, such as U.S. Pat. No. 4,703,342, U.S. Pat. No. 4,954,885, U.S. Pat. No. 5,225,899, U.S. Pat. No. 5,231,477, U.S. Pat. No. 5,231,478, U.S. Pat. No. 5,386,244, U.S. Pat. No. 5,416,531, U.S. Pat. No. 5,416,532, U.S. Pat. No. 5,517,255, and EP0161923A2, have disclosed different methods and apparatus to separate the luminance signal from the video signal.
FIG. 1 is a function block diagram of a video signal separator 10 according to the prior art. The video signal separator 10 is capable of separating an analog video signal into a luminance signal and a chrominance signal. The video signal separator 10 includes an analog/digital converter (ADC) 12 for converting the analog video signal into a digital downward reference video signal DRVS, a first line delay circuit 14 electrically connected to the ADC 12 for delaying the DRVS by one horizontal line (will be described in more details later) so as to generate a target video signal TVS, a second line delay circuit 16 electrically connected to the first line delay circuit 14 for delaying the TVS by one horizontal line so as to generate an upward reference video signal URVS, a horizontal reference chrominance signal generator 18 (HRCS generator) for receiving the DRVS, the TVS and the URVS so as to generate a horizontal reference chrominance signal HRCS, a vertical reference chrominance signal generator 20 (VRCS generator) for receiving the TVS so as to generate a vertical reference chrominance signal VRCS, a horizontal/vertical reference chrominance signal generator 22 (HVRCS generator) for receiving the DRVS, the TVS and the URVS so as to generate a horizontal/vertical reference chrominance signal HVRCS, a judging device 24 for receiving the DRVS, the TVS and the URVS so as to generate a select signal, a multiplexer 26 for outputting one of three signals HRCS, VRCS and HVRCS by determining the select signal transmitted from the judging device 24, and a subtractor 28 for subtracting signals output by the multiplexer 26 from the TVS so as to generate the luminance signal, and a signal output from the multiplexer 26 being the chrominance signal.
The ADC 12 samples the analog video signal with a sampling frequency fs, which is four times a color subcarrier fsc, and generates the DRVS, so chrominance signals of two adjacent sampling points have ninety degrees of phase difference. Since the color subcarrier fsc is equal to (455/2)fH, wherein fH is a horizontal scan frequency, which is equal to the amount of horizontal scan lines projected onto a screen 40 per second by a cathode tube of a color TV, chrominance signals of any sampling points on two adjacent horizontal scan lines have 180 degrees of phase difference. FIG. 2 illustrates a phase diagram of chrominance signals of any sampling points of three adjacent horizontal scan lines on screen 40 according to the prior art. A first line 42, a second line 44 and a third line 46 each include five sampling points, and an arrow embraced in a sampling point indicates a phase of the sampling point. For example, three sampling points 48, 50 and 52 are respectively represented by Y+C, Y−C and Y+C, wherein Y represents a luminance signal of a sampling point while C represents a chrominance signal of a sampling point, and chrominance signals of the sampling points 48 (Y+C) and 50 (Y−C) have 180 degrees of phase difference.
Operations of the video signal separator 10 are described as follows: Please refer to FIG. 1 again. When a video signal of an NTSC (National Television Standards Committee) system travels through the ADC 12, the ADC 12 transforms the video signal into the DRVS with the sampling frequency fs. Then the first line delay circuit 14 delays the DRVS by a horizontal scan line so as to generate the TVS. The second line delay circuit 16 delays the TVS by a horizontal scan line and generates the URVS. The HRCS generator 18, the VRCS generator 20 and the HVRCS generator 22 respectively transform the DRVS, the TVS, and the URVS into the HRCS, the VRCS, and the HVRCS. For example, a transfer function for the HRCS generator 18 can be Ch(Z)=(−1/4)(1−Z−2)2, a transfer function for the VRCS generator 20 can be Cv(Z)=(−1/4)(1−Z−L)2, and a transfer function for the HVRCS generator 22 can be Chv(Z)=(−1/4)(1−Z−2)2(−1/4)(1−Z−L)2, wherein Z−1 represents delaying a sampling point while Z−L represents delaying a horizontal scan line. Then the multiplexer 26 determines the select signal transmitted from the judging device 24 and outputs one of the HRVS, the VRCS and the HVRCS. How the judging device 24 generates the select signal is described as follows.
FIG. 3 illustrates a function block diagram of the judging device 24 according to the prior art. The judging device 24 includes a horizontal luminance signal non-correlated value calculator 60 (HLNC calculator) for calculating a horizontal luminance signal non-correlated value HLNC, a horizontal chrominance signal non-correlated value calculator 62 (HCNC generator) for calculating a horizontal chrominance signal non-correlated value HCNC, a vertical luminance signal non-correlated value calculator 64 (VLNC generator) for calculating a vertical luminance signal non-correlated value VLNC, and a vertical chrominance signal non-correlated value calculator 66 (VCNC generator) for calculating a vertical chrominance signal non-correlated value VCNC, and a judging circuit 68 for generating the select signal according to the HLNC, the HCNC, the VLNC and the VCNC. The judging circuit 68 determines a smallest non-correlated value among these four non-correlated values and generates the select signal to guide the multiplexer 26 to output the HRVS, the VRCS or the HVRCS according to the smallest non-correlated value. The aforementioned calculator 60, 62, 64 and 66 are all used to calculate non-correlation among a plurality of signals. The non-correlation calculation is well known by those skilled in the art, and a detailed description is omitted here.
As the judging device 24 generates the select signal, the multiplexer 26 is capable of generating the chrominance signal according to the select signal. Then the subtractor 28 calculates the luminance signal by subtracting the chrominance from the video signal, that is, separating the luminance signal from the video signal.
The video signal separator 10 of the prior art has at least two drawbacks:
1. The judging device 24 of the video signal separator 10 depends on only three video signals (the DRVS, TVS and URVS) to generate the select signal. That the judging device 24 depends on only the TVS and two video signals respectively disposed upward and downward next to the TVS to generate the select signal is likely to generate a non-appropriate select signal and further calculates a false luminance signal and chrominance signal as well. For example, if the differences from the TVS to the upward as well as to the downward video signal are both significantly large while these two differences differs slightly from each other, the VLNC calculator 64 of the judging device 24 calculates a small VLNC and the judging circuit 68 transmits corresponding select signal to the multiplexer 26 according to the small VLNC, misleading the multiplexer 26 to output false luminance signal and chrominance signal.
2. The video signal separator 10 of the prior art generates the luminance signal as well as the chrominance signal in a so-called hard decision way—the multiplexer 26 only outputs one of the HRVS, TVS and VRVS as the luminance signal. When the non-correlated values respectively generated by the calculators 60, 62, 64 and 66 differs significantly from one another, the hard decision can calculate a luminance signal and a chrominance signal both with a tolerable error. However, when the non-correlated values generated by the calculators 60, 62, 64 and 66 are not significantly differ from one another, the select signal output from the judging device 24 by referring to these non-correlated values may mislead the multiplexer 26 to output improper luminance signal and chrominance signal.